Northern Lights: Magnetic Fields
Magnetic Fields are responsible for much of the activity on the Sun. The hot plasma on the Sun and in Coronal Mass Ejections (CMEs) consists of charged particles that are strung along loops of magnetic field lines that can bend, twist, and break like rubber bands. Our own Earth is surrounded by a huge magnetic field called the magnetosphere, which interacts with and can trap the charged particles from the Sun, causing dramatic space weather.
- Magnetic Fields
- Magnetic Reconnection
You are probably familiar with magnets that go on your refrigerator, or maybe you have played with magnets and found that “opposites attract”. For example, south magnetic poles are attracted to north magnetic poles but are repelled by other south poles. Every magnet has a south pole and a north pole, and cutting a magnet in two only makes two more magnets, each with its own south pole and north pole.
A North Magnetic pole is defined as one that points to or is attracted to the Earth's North pole. Since opposites attract this means that the Earth's North pole is actually defined as its magnetic South pole!
Compasses are magnetized. They always point North because their South poles want to align with the Earth's magnetic North pole.
Magnets can affect things that they aren't touching. For example, if you hold a refrigerator magnet close to the refrigerator, but without touching it, you can feel that you have to pull a little bit so that the magnet does't jump to the refrigerator door. We call this extended effect of the magnet its "field."
If we place a compass at different points around a magnet, we can see that it points in different directions. Checking more places shows that there are clear paths that the field follows. In fact, it always points out from the North pole, looping out and around back to the South pole, and connecting inside the magnet.
Magnetic fields are always loops. They never begin or end at any one particular spot.
A bunch of iron fillings make the magnetic field more clear. You can see the field is stronger close to the magnet (iron fillings are bunched up) and weaker far away (iron fillings more sparse).
Magnetic field lines can bend, twist, stretch, and even break, just like a rubber band. Sometimes a looping field can stretch so much that it pinches off and breaks, reconnecting with other nearby magnetic field lines and leaving a free loop of magetic field lines.
The animation above shows how this can happen on the Sun. You can see the magnetic loops beneath the surface puncture the surface, causing the dark sunspots of intense magnetic field. As the loop rises, it stretches and flails - this is the flare. The loop pinches off and breaks, then reconnects to nearby field lines. This releases a huge amount of energy that launches the "plasmoid" (freed magnetic loops with the plasma connected to them) out into space, which is what we call a coronal mass ejection (CME). This is the material that can reach Earth and cause dramatic space weather, including the Aurorae.
Magnetic Reconnection can also occur in the Earth's magnetosphere. A strong CME can push the night-side tail of Earth's magnetosphere far away, stretching it and causing it to snap and break. The reconnecting field lines catch some of the charged particles from the CME and funnel them along the magnetic field lines back to the North and South magnet poles. When these particles encounter the atmosphere, they excite oxygen and nitrogen atoms, creating the beautiful colors of the Aurorae.
For more information about magnetic reconnection, check this website: Magnetic Reconnection Overview.